Polyarylene sulfides (hereafter abbreviated as “PAS”), representative examples of which are polyphenylene sulfides (hereafter abbreviated as “PPS”), are engineering plastics exhibiting excellent heat resistance, chemical resistance, flame retardancy, mechanical strength, electrical characteristics, dimensional stability, and the like. PAS are frequently used in a wide range of fields such as electrical/electronic equipment and automobile equipment due to their moldability into various molded products, films, sheets, fibers, and the like by general melt processing methods such as extrusion molding, injection molding, and compression molding.
A known example of a representative method for manufacturing PAS is a method of reacting a sulfur source and a dihalo aromatic compound in an organic amide solvent such as N-methyl-2-pyrrolidone (hereafter abbreviated as “NMP”). A PAS obtained by this method typically tends to have a structure in which a halogen bonds to the terminal of a polymer and therefore has a high halogen content, even when sufficiently washed in the separation/recovery step after a polymerization reaction. When such a PAS having a high halogen content is used, environmental pollution becomes a problem, as evidenced by halogen regulations in recent years. In ordinary polymerization without phase separation, PAS is mostly separated/recovered as a fine powder after the polymerization reaction, resulting in poor handleability. In addition, a polymer produced during the polymerization reaction becomes less soluble in the organic amide solvent as polymerization progresses, which makes growth response difficult. As a result, it is difficult to obtain a polymer having the targeted melt viscosity with a low halogen content, so there is still a demand for improvement. In order to improve upon the problems described above, a method of performing a polymerization reaction in the presence of a phase separation agent has been developed as a manufacturing method. However, this method has not yet yielded a PAS having satisfactory performance.
In many fields such as the field of electrical/electronic equipment in recent years, PAS have come to be widely used as compounds into which fillers such as glass fibers, for example, are blended. Such compounds ordinarily contain approximately 30 to 50 mass % glass fibers, and since the compounds are used in the field of electrical/electronic equipment or the like, there is not only the problem of reducing the halogen content from the perspective of environmental regulations, but there are also strong demands for a simple molding process. As a means for solving this problem, there is a demand for a PAS having good thermal stability, low gas generation at the time of molding processing, and low melt viscosity. This is because if the melt viscosity of the PAS is high at the time of the melt molding of such a compound, heat degradation of the PAS tends to occur due to localized temperature increases resulting from friction caused by kneading with hard glass fibers. As a result, the thermal stability becomes poor, and the amount of gas generated increases, which causes problems such as the inability to achieve stable and favorable melt molding conditions.
However, when the molecular weight is simply reduced in order to reduce the melt viscosity of the PAS, a PAS with a high halogen content is produced, which is against a reduction in the halogen content. This is presumed to be due to the fact that the number of molecules of the PAS increases with a reduction in molecular weight, and as a result, the number of PAS molecular terminals increases, so the number of PAS molecular terminals to which halogen bonds also increases. In this way, a PAS with a low melt viscosity (that is, a PAS with a low molecular weight) inevitably has a higher halogen content than that of a PAS with a high molecular weight due to a larger number of polymer terminals. In other words, a reduction in the melt viscosity (increase in fluidity) and a reduction in halogen have an antinomic relationship. However, as described above, in the field of electrical/electronic equipment in recent years, there is an increasing demand for regulations to ensure reduced halogen out of environmental considerations, and there is an increasing demand for a PAS having good thermal stability, a low halogen content, and a low melt viscosity.
Japanese Unexamined Patent Application Publication No. 2010-126621A (Patent Document 1) proposes the idea of adding one or more types of compounds selected from the group consisting of mercapto compounds, metal salts of mercapto compounds, phenol compounds, metal salts of phenol compounds, and disulfide compounds in order to obtain a PAS with a low halogen content.
However, in Working Examples 1 to 6 of this Patent Document 1, which use a thiophenol (Working Example 7 uses a phenol), the improvement in chlorine content is limited to approximately 1,200 to 2,100 ppm. In addition, in Working Example 8, which is the only working example using a disulfide compound (diphenyl disulfide) in Patent Document 1, it is reported that the chlorine content was 1,800 ppm, which is still a high chlorine content. Furthermore, as evidenced by the description in Patent Document 1 that “powder-like PAS was obtained” in Working Example 1, the produced PAS is likely in the form of a fine powder rather than a granular form. In order to recover this fine powder-like PAS, a method of recovering the substance by filtration is adopted, but an oligomer with a high chlorine content is also recovered, as described below, which not only makes it impossible to sufficiently reduce the chlorine content, but also leads to the problem that it is difficult to obtain a PAS having good thermal stability and low gas generation at the time of molding processing. Furthermore, the recovery rate of the powder-like PAS obtained by filtration is low at 95% in Working Example 1.
Similarly, in Japanese Unexamined Patent Application Publication No. 559-215323A (Patent Document 2), which uses disulfides, the yield of a PAS obtained by filtration in the case of Working Example 16 using diphenyl disulfide is low at 90.5%.
Furthermore, the thiophenol used in Patent Document 1 is easily oxidized when held in storage or handled at the time of production. Therefore, when producing the substance industrially, fluctuations arise in the molecular weight of the PAS or the halogen (chlorine) reducing effect depending on the degree of oxidation, and industrial production within constant product standards will be difficult.
In addition, since thiophenol or the like has a foul odor, environmental problems arise in the production step and recovery step. Furthermore, there are cases in which the PAS that is produced is also contaminated with the foul odor.
Furthermore, when a sulfur source and a dihalo aromatic compound are polymerized in an organic amide solvent in the presence of an alkali metal hydroxide, the organic amide solvent such as NMP and the alkali metal hydroxide react due to heating, and a compound containing the nitrogen element is produced as an impurity. For example, when NMP and sodium hydroxide (NaOH) are reacted, NMP is subjected to ring opening, and sodium methylamino butanoate [(CH3)NH—CH2—CH2—CH2—COONa] is produced. This compound reacts with p-dichlorobenzene, which is a dihalo aromatic compound, to produce sodium chlorophenyl methylamino butanoate. Furthermore, these may be uptaken into the polymer terminals at the time of the PAS polymerization reaction. The contamination of the metal mold, die, or the like caused by such a compound containing nitrogen atoms has an adverse effect on the quality of the molded product, so the need to frequently clean the metal mold, die, or the like arises.
Accordingly, it is important for the nitrogen content to be reduced in the PAS that is produced.
In this way, it is difficult to efficiently obtain a granular PAS having good thermal stability, low gas production at the time of molding processing, low melt viscosity, and reduced nitrogen content while reducing the halogen content with conventional technology, and there has been a strong demand for improvements.